When it comes to Internet privacy, corporate lobbying goes only so far

FORTUNE — Despite hardball lobbying and vast piles of money shoved toward members of Congress, it’s proving to be extremely difficult to pass legislation that is opposed by advocates of privacy and Internet freedom. This was proved last year with the defeats of anti-piracy measures SOPA and PIPA, and the same might be true again with weakening support for CISPA — the Cyber Intelligence Sharing and Protection Act.

That proposed law would allow (but not require) companies to share personal information with the government without fear of liability under existing privacy laws. Some companies that had originally supported the bill have backed down: Microsoft last April, and Facebook just this month.

The about-faces of those two companies came amid a flurry of lobbying from such diverse groups as the Electronic Frontier Foundation, the American Civil Liberties Union, and the Republican Liberty Caucus. Another group, Demand Progress, had targeted Facebook FB, and particularly its CEO, Mark Zuckerberg, with a letter-writing campaign.

Lots of other companies and lobbying groups still support CISPA, though, including tech, telecom and cable giants like Verizon VZ, AT&T T, IBM IBM, Oracle ORCL, and Comcast CMCSA, as well as lobbyists such as the U.S. Chamber of Commerce, the American Bankers Association, and the Telecommunications Industry Association. Opposition to the bill is overwhelmingly composed of public-interest groups.

In the two years ending last June, pro-CISPA companies and organizations contributed 13 times more money to the campaigns of House members than did the anti-CISPA groups: $55 million vs. $4 million, according to data published this week by MapLight, an organization that tracks lobbying spending. The gap is even wider — supporters donated 15 times more money than opponents — when it comes to the campaigns of members of the House Intelligence Committee, where the bill originated.

The pro-CISPA forces might need to exert a lot more muscle than they already have, though: President Obama has promised to veto the bill — which was already rejected once by the Senate after passing the House last year — should it reach his desk.